Rivista Italiana di Paleontologia e Stratigraia (Research in Paleontology and Stratigraphy) vol. 123(2): 255-273. July 2017 SEQUENCE STRATIGRAPHY AND PALEONTOLOGY OF THE UPPER MIOCENE PISCO FORMATION ALONG THE WESTERN SIDE OF THE LOWER ICA VALLEY (ICA DESERT, PERU) CLAUDIO DI CELMA1*, ELISA MALINVERNO2, GIULIA BOSIO2, ALBERTO COLLARETA3,4, KAREN GARIBOLDI3,4, ANNA GIONCADA3, GIANCARLO MOLLI3, DANIELA BASSO2, RAFAEL M. VARAS-MALCA5, PIETRO PAOLO PIERANTONI1, IGOR M. VILLA2, OLIVIER LAMBERT6, WALTER LANDINI3, GIOVANNI SARTI3, GINO CANTALAMESSA1, MARIO URBINA5 & GIOVANNI BIANUCCI3 1*Corresponding author. Scuola di Scienze e Tecnologie, Università di Camerino, Camerino, Italy. Email: [email protected] 2Dipartimento di Scienze dell’Ambiente e della Terra, Università di Milano-Bicocca, Milano, Italy 3Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy. 4Dottorato Regionale in Scienze della Terra Pegaso, Pisa, Italy. 5Departamento de Paleontología de Vertebrados, Museo de Historia Natural, Universidad Nacional Mayor de San Marcos, Lima, Peru. 6D.O. Terre et Histoire de la Vie, Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium. To cite this article: Di Celma C., Malinverno E., Bosio G., Collareta A., Gariboldi K., Gioncada A, Molli G., Basso D., Varas-Malca R.M., Pierantoni P.P., Villa I.M., Lambert O., Landini W., Sarti G., Cantalamessa G., Urbina M. & Bianucci G. (2017) - Sequence stratigraphy and paleontology of the Upper Miocene Pisco Formation along the western side of the lower Ica Valley (Ica Desert, Peru). Riv. It. Paleontol. Strat., 123(2): 255-273. Keywords: Pisco Formation; late Miocene; sequence stratigraphy; vertebrate and macro-invertebrate paleontology; diatom biostratigraphy; tephrochronology. Abstract. The sequence stratigraphic framework and a summary of the fossil fauna of the Upper Mioce- ne portion of the Pisco Formation exposed along the western side of the Ica River (southern Peru) is presented through a new geological map encompassing an area of about 200 km2 and detailed chronostratigraphic analyses. Extensive ield mapping and sedimentological study of outcrop sections have shown that the Pisco Formation is a cyclical sediment unit composed of at least three ining-upward, unconformity-bounded depositional sequences, designated P0, P1, and P2 from oldest to youngest. In the study area, these sequences progressively onlap a com- posite basal unconformity from southwest to northeast. Integration of biostratigraphic and tephrochronologic age determinations constrains the ages of the three Pisco sequences within the study area. Based on the age of surrounding sediments, a conservative estimate of the age of P0 suggests deposition of these strata between 17.99 ± 0.10 Ma and 9.00 ± 0.02 Ma, whereas diatom biostratigraphy and calculated 40Ar/39Ar ages converge to indicate that strata of the P1 sequence were deposited sometime between 9.5 Ma and 8.9 Ma and that those of the P2 sequence are younger than 8.5 Ma and older than 6.71 ± 0.02 Ma. Our survey for both vertebrate and macro-invertebrate remains in the three sequences conirms the outstanding paleontological value of the Pisco Formation and contributes to depict regional faunal shifts in the fossil assemblage. INTRODUCTION et al. 2017), and mollusks (DeVries 2007). The ac- curate placement of these discoveries into a proper Over the past decades, the extensive paleontological stratigraphic context, however, remains a largely un- investigations that have been conducted on the Up- resolved issue that bears upon the general lack of per Miocene to Pliocene Pisco Formation exposed in robust biostratigraphic and tephrochronologic age the Ica Desert, southern Peru, have yielded substan- constraints and uncertain intraformational correla- tial assemblages of exquisitely preserved fossil marine tions (Brand et al. 2011). As a result, the paleontologi- vertebrates, including cetaceans, pinnipeds, croco- cal research has mostly focused on systematic studies diles, seabirds, sharks, and bony ish (e.g., Bianucci ofet single skeletons, the ecology of fossil vertebrates, al. 2016a, 2016b, 2016c; Stucchi et al. 2016; Landini and the processes governing their preservation, abun- dance, and taphonomy (Muizon 1988; Brand et al. 2004; Esperante et al. 2008, 2015; Lambert et al. 2009, Received: September 06, 2016; accepted: March 20, 2017 2010a, 2010b, 2014, 2015, 2017a, 2017b; Bianucci et 256 Di Celma C. et al. brates and the biological and physical factors dictating their distribution and evolutionary trends. REGIONAL TECTONICS AND STRATIGRAPHIC BACKGROUND Geological setting. The regional tectonic setting of the Peruvian margin is dominated by the oblique subduction of the Nazca/Farallon Plate un- derneath the South American Plate in the Peru-Chile Trench. This subduction, which began in the Creta- ceous and continues today, developed a composite transform-convergent margin characterized by nor- mal and strike-slip faults (e.g., Barazangi & Isacks 1979; Pilger 1981; Cahill & Isacks1992; León et al. 2008; Zúñiga-Rivero et al. 2010). According to Thornburg & Kulm (1981), two long and narrow structural ridges, the Outer Shelf Ridge and the Upper Slope Ridge, were formed on the con- Fig. 1 - Sketch map of the major sedimentary basins of coastal Peru tinental shelf and upper slope of the Peruvian con- showing position of both the Outer Shelf Ridge and Upper Slope Ridge, redrawn and modiied from Travis, Gonzales, tinental margin by late Cretaceous – early Paleogene & Pardo (1976) and Thornburg & Kulm (1981). tectonic activities that involved Paleozoic and Meso- zoic strata. These two ridges and associated shorter, transverse shelf uplifts, or local highs, subdivided al. 2010, 2016a; Collareta et al. 2015, 2017; Gariboldi the Peruvian offshore into a series of Cenozoic sedi- et al. 2015; Gioncada et al. 2016). By contrast, correla- mentary basins (Fig. 1). In coastal southwestern Peru, tions of individual fossil indings to temporally con- the most westerly Upper Slope Ridge separates the strained measured sections have only been established Pisco basin into an offshore and an onshore portion, for two important fossil-bearing localities (Bianucci et referred to as the West Pisco basin and East Pisco al. 2016b, 2016c), where comprehensive stratigraphic basin, respectively. Rapid uplift and emersion of the frameworks were recently established (Di Celma et al. Cenozoic ill and underlying composite basement of 2016a, 2016b). the East Pisco basin is interpreted to be related to The primary objectives of this paper, which is focused subduction of the aseismic Nazca Ridge beneath this on the portion of the Pisco Formation exposed south part of Peruvian forearc during the Quaternary (e.g., of the Ocucaje village, along the western side of the Hsu 1992; Macharé & Ortlieb 1992; von Huene et al. Ica River, are: i) to set the stratigraphic cyclicity evident 1996; Hampel et al. 2002). in the architecture of the Pisco Formation within a sequence stratigraphic framework; ii) to constrain the Stratigraphic overview. The sedimentary ill age of the three unconformity-bounded depositional of the East Pisco basin has been described by Dunbar sequences through the integration of radiometric dat- et al. (1990) and DeVries (1998) and consists of four ing of volcanic ash layers and diatom biostratigraphy; lithostratigraphic units: the middle to upper Eocene and iii) to provide a summary of the vertebrate and Paracas Formation, the uppermost Eocene-lower macro-invertebrate fossil assemblages found within Oligocene Otuma Formation, the uppermost Oligo- these Upper Miocene marine strata. These develop- cene to lower Miocene Chilcatay Formation, and the ments represent a irst step infacilitating correlation Upper Miocene to Pliocene Pisco Formation. These between scattered and geographically disparate out- units are underlain by a deformed and heterogeneous crop sections and provide a sound basis for future basement including lower Paleozoic intrusive rocks studies on the systematics, comparative morphology, and Jurassic volcano-sedimentary rocks. The above phylogeny and paleoecology of Pisco fossil verte- mentioned Cenozoic formations are bounded by Upper Miocene Pisco Formation (Ica Desert, Peru) 257 regionally extensive, conglomerate-mantled uncon- RESULTS formities that are locally accompanied by angular discordances and document relatively long breaks in Cenozoic stratigraphy. The Cenozoic strati- sedimentation. graphic record exposed across the study area com- prises two distinct formations, namely the Chilcatay and Pisco formations. Close to the basin margins, STUDY AREA AND METHODS these two units rest nonconformably onto a paleo- surface carved into a complex basin basement in- The detailed stratigraphic and mapping work cluding lower Paleozoic gabbroic to granitoid rocks of the Pisco Formation provides the necessary con- of the San Nicolás batholith and Jurassic volcano- text for the sequence stratigraphy, chronostratigra- sedimentary rocks of the Guaneros Formation (Mu- phy, and paleontology presented in this study. It was kasa & Henry 1990; León et al. 2008). conducted over the course of three ield campaigns between 2015 and 2016 and encompasses an area of Chilcatay Formation. approximately 200 km2 that stretches for 22 km along The lower portion of the Chilcatay Formation the western side of the Ica River valley, from Cerro comprises brownish to light coloured, ine- to Blanco in the north to Cerro las Tres Piramides in the very coarse-grained biocalcarenite